1. Field of the Invention
The present invention relates to an apparatus for transmitting and receiving signals in a wireless telecommunication system and a method for transmitting random access bursts in such an apparatus. Particularly, the present invention relates to a backoff scheme for the transmission of random access bursts in a mobile station of a wireless telecommunication system.
2. Description of the Related Art
In wireless telecommunication systems, as for example the GSM system, the mobile station uses the random access channel (RACH) to send random access bursts to acquire services from the network. If for example a mobile station of a mobile cellular system needs a data channel to transmit user data, it sends a random access burst over the random access channel to the selected base station. The base station receiving a random access burst sends an acknowledgement or a confirmation message to the mobile station to indicate, that the transmission of the random access burst from the mobile station to the base station has been successful. The confirmation information might be the access grant information sent by a base station to indicate that the requested service is granted. After a successfully transmitted random access burst, the base station allocates the respective requested service, for example a user data channel, to the mobile station.
The transmission of a random access burst from a mobile station to a base station is contention based and collisions can occur. If for example several mobile stations within one cell of the telecommunication system simultaneously send random access bursts, collisions can occur and the random access bursts can not be correctly received and confirmed by the base station. Having sent a random access burst to a base station, the mobile station checks if an acknowledgement information from the base station has been received. If an acknowledgement information has not been received after a predetermined time period, the mobile station decides that the transmission attempt of the random access burst has not been successful. After such a failed random access burst transmission attempt, a backoff mechanism determines, when a new transmission of the random access burst shall take place. The same is true after a retransmission of a random access burst and not only for the first transmission attempt.
Different kinds of backoff mechanisms are known in the prior art. For example, in a so-called bayesian backoff scheme, the network side, i.e. the base station, estimates, if the random access channel has been occupied or not, if collisions occurred or if a random access burst has been successfully transmitted. Estimation information about the result of this estimation is sent to the mobile stations, so that the mobile stations can set their backoff mechanisms depending on the estimation information. Another known possibility is a detection of the collisions of competing random access bursts at the user equipment side. The known systems require much downlink resources, since the base station has to regularly transmit estimation information to the mobile stations or are very complex because collision detection is needed.
The U.S. Pat. No. 5,544,196 discloses an apparatus and a method for reducing message collision between mobile stations simultaneously accessing the base station in a wireless telecommunication system. Hereby, a mobile station inserts a random delay between successive random access burst transmissions if it does not receive an acknowledgement after a predetermined time period. The random delay is set by randomly choosing a time point from a predetermined time interval. Thus, the time interval, from which a new time point for a retransmission of a random access burst is chosen, is fixedly preset. This relatively simple backoff mechanism is also used in the GSM system. Particularly in cases, in which the random access channel is very occupied and a lot of collisions occur, this known system is very ineffective, since a lot of random access burst transmission attempts are performed with a very low probability of a successful transmission.